1. Field of the Invention
The present invention relates to a method of measuring ion activity for the quantitative analysis of specific ion activity (or ion concentration) contained in an aqueous liquid, particularly a body fluid such as blood, urine or saliva, utilizing potentiometry, and a measuring device employable in said method
2. Description of Prior Arts
There has been known a method of measuring specific ion activity contained in a liquid sample of an aqueous liquid (e.g., tap water, river water, sewage or industrial drainage) and a body fluid (e.g., blood such as whole blood, plasma and serum; urine, or saliva) by using an ion activity measuring device in the form of sheet.
In the method, a reference liquid and a liquid sample are spotted onto surfaces of ion-selective membranes, respectively, which are arranged on the top of at least a pair of ion-selective electrode sheets, and then potential difference between the ion-selective electrodes is measured under the condition that both liquids are electrically connected to each other by means of a bridge, so as to determine the ion activity of the liquid sample.
Examples of the ion activity measuring device employable in the method are described, for instance, in Japanese Patent Provisional Publications No. 52 (1977)-142586 (corresponding to U.S. Pat. No. 4,053,381), No. 56(1981)-6148 (corresponding to U.S. Pat. No. 4,273,639) and No. 58(1983)-211648 (corresponding to U.S. Pat. No. 4,437,970). In these devices, a pair of ion-selective electrode sheets are arranged in such a manner that ion-selective membranes are positioned on the upper side, and on the ion-selective membranes are provided liquid receiving openings (openings for allowing introduction of a reference liquid and a liquid sample). In practically determining the ion activity by the use of those devices, the reference liquid and liquid sample are spotted onto the ion-selective membranes through the liquid receiving openings using a pipet, etc., and a potential difference between both ion-selective electrodes is measured. As an improved device, there is known a device comprising a plurality of pairs of ion-selective electrodes, which can determine ion activity of plural kinds of ions by only once spotting of a reference liquid and a liquid sample thereonto.
The above-described method using at least one pair of ion-selective electrode sheets is an easy and advantageous method for determining ion activity, but the present inventors have found that there are various problems not only in the preparation of the measuring device but also in the measuring operation.
For instance, as the first problem, an ion-selective membrane of the ion-selective electrode is easily damaged by a tip of a pipet which is generally employed for spotting a reference liquid or a liquid sample. Since the ion-selective membrane is not solid but like a jelly, the membrane is easily damaged or distorted when the tip of pipet comes into contact with the surface of the membrane. Such deterioration of the shape of the ion-selective membrane brings about an error in the determination of ion activity, and in an extreme case, the determination thereof becomes impossible.
As the second problem, a silver chloride layer of the ion-selective electrode is easily deteriorated. Most of the ion-selective electrodes utilize a silver/silver chloride electrode (i.e., half cell) as an inner reference electrode. In this case, if the ion activity measuring device is allowed to stand in a light room, the silver chloride layer is denatured by a light entering from the liquid receiving opening, whereby the electrode is deteriorated.
As the third problem, a probe which serves to measure the potential difference occurring between the ion-selective electrodes is apt to be stained. An ion-selective electrode sheet is generally employed in a form comprising a plastic sheet support and an electrode placed on the surface thereof. In determining the ion activity by the use of the ion-selective electrode sheet, it is necessary to measure the potential difference between a pair of ion-selective electrodes as described hereinbefore. Accordingly, at the ends of the electroconductive portion of the ion-selective electrode (e.g., a silver layer of a silver/silver chloride electrode) are provided electricity-connecting regions (e.g., elongations of the silver layer). The electricity-connecting regions are brought into contact with the aforementioned probes of the potential difference measuring device which is prepared separately so as to determine the potential difference. When the ion-selective membrane is placed on the upper side as in the conventional device, the surface of the electricity-connecting region naturally faces to the upper side, and the probe of the potential difference measuring device is brought from the upper side into contact with the surface of the electrically connecting region. Accordingly, the probe tends to be stained with the reference liquid or liquid sample which is introduced from the same upper side.
As the fourth problem, the measuring system of the potential difference measuring device used in combination with the ion activity measuring device becomes complicated. In more detail, if the ion-selective membrane is placed on the top, the surface of the electrically-connecting region faces to the upper side, and accordingly the probe of the potential difference measuring device is necessarily arranged to contact on the upper side with the surface of the electrically connecting region, as described above. In such arrangement, in order to obtain smooth electric wiring between the probe of the potential difference measuring device and an amplifier such as a head amplifier settled in the vicinity of the prove, the electric wiring is required to be made on the upper side of the device or in the side direction of the device. However, it is generally advantageous to arrange the probe of the potential difference measuring device below the ion activity measuring device, from the viewpoint of preventing the probe from the contact with the reference liquid or liquid sample or saving the space required for settling the devices. In utilizing the conventional ion activity measuring device which is formulated to be used in the system that an ion-selective membrane is placed on the upper side and a reference liquid or a liquid sample is spotted on the membrane, the measuring system of the potential difference measuring device used in combination with the ion activity measuring device becomes complicated, and a large space is required for the measuring system.
As the fifth problem, there is a difference between times required for a reference liquid or a liquid sample reaching the surface of each electrode in measuring plural kinds of ion activity using the conventional measuring system. Further, there is also a difference between spotting areas. Said differences are factors of errors in measurement.